Concept:Rhizobium and leguminous plants live together and both gain benefits, forming a symbiotic relationship.Explanation:Rhizobium bacteria live in the root nodules of leguminous plants.They convert atmospheric nitrogen into usable nitrogen compounds for the plant.In return, the plant supplies the bacteria with carbohydrates and shelter.Because both organisms benefit from this association, it is known as mutualism.Answer:C. mutualism